The invention relates to a method for electrochemical machining of an electrically conductive work piece using an electrochemical machining device including a tool electrode opposing the work piece across a predetermined machining gap filled with electrolyte, the electrochemical machining device further comprising means for supplying machining voltage pulses across the machining gap.
The invention also relates to an arrangement for electrochemically machining an electrically conductive work piece by applying electrical machining pulses between the work piece and an electrically conductive electrode while electrolyte is supplied between the work piece and the electrode.
Electrochemical machining is a process in which an electrically conducting work piece is dissolved at the location of an electrode while electrolyte and electric current is supplied. For this purpose, the electrode is brought in the proximity of the work piece and, while electrolyte is fed into the gap between the work piece and the electrode, a powerful current is passed through the work piece and the electrode via the electrolyte, the work piece being positive with respect to the electrode. The current is applied in the form of machining pulses having a given amplitude and duration. In the intervals between the machining pulses, the electrolyte is renewed. Under the working conditions the work piece is being dissolved, thus increasing the value of the gap between the work piece and the electrode. To compensate for this, the electrode and the work piece are moved towards one another with a given feed rate, as a result of which the electrode forms a cavity or eventually a hole in the surface of the work piece, the shape of the cavity or hole having the shape corresponding to the shape of the electrode. This process can be used, for example, for making intricate cavities or holes in or for shaping hardened metals or alloys. The copying precision with which the shape of the cavity or the hole in the work piece corresponds to the shape of the electrode is important for the quality of the result.
A method for electrochemical machining is known from a patent application Publication WO 99/51382. According to the known method, in the intervals between the machining pulses and the passivation pulses for depositing passivation layers on the work piece are applied deliberately. By selecting a proper amplitude and duration of the passivation pulses, the spatial distribution of the passivation layer can be controlled. It is preferable to obtain the passivation layer with higher thickness at the lateral surfaces of the obtained cavity with respect to the passivation layer thickness at the front surface of the cavity. In this case, the dissolution rate at the front surface will be higher with respect to the lateral surfaces, which leads to a better copying accuracy.
The drawback of the known method for improving the copying accuracy is the difficulty associated with the selection of the values for the passivation pulse characteristics as well as gap dimensions with respect to both front and lateral surfaces of the cavity for obtaining a non-uniformly distributed passivation layer. The forming of the passivation layer is influenced by the local electrical field strength. Due to the field in-homogenities caused by the electrode curvature as well as by the precipitations on the cathode surface it is not possible to create the operational condition for an optimal copying accuracy.
It is an object of the invention to provide an electrochemical machining method with a further improved copying precision, where the process control can be optimized. To this end, the method of the present invention involves an application of a first number of the machining voltage pulses of predetermined optimal duration across the machining gap that is alternated with a second number of measurement voltage pulses across the machining gap in order to measure an actual value of the machining gap.
According to the technical measure according to the invention and based on a fundamental insight of the electrochemical processes in the gap, for each predetermined value of the gap there is a single optimal pulse duration, corresponding to the optimal local copying accuracy. It is understood that, for example in case of the adjacent cavities having different respective depths, the maximum copying precision can be achieved in case the local dissolution efficiencies vary substantially. Such an optimal operating condition is valid for a certain value of the gap. By alternating the machining pulses with measurement pulses it is possible to obtain an accurate information about the gap dimensions on-line during the electrochemical machining. In case the measurement of the gap dimensions shows a value deviating from a preset value, it is possible to alter the operating conditions by bringing the gap back to the predetermined value or by selecting another machining pulse duration, corresponding to the optimal pulse duration for the measured actual value of the gap. It is important to mention, that in case the measurement of the value of the gap positively deviates from the predetermined value, it is preferable to set the system back to the optimal operating conditions by reducing the value of the gap back to the predetermined value.
An embodiment of the method according to the invention is characterized in that the optimal duration of the machining voltage pulses is derived from the maximum value of a localization coefficient for the predetermined value of the machining gap. This technical feature is based on the insight that in case of adjacent cavities the maximum localization coefficient, corresponding to a ratio of local dissolution rates, results in the optimal copying precision. It is further understood that the local anodic dissolution rate is given by the local value of the current density, leading to the conclusion that the localization coefficient (L) is given by the ratio of local current density values J(xcfx84,si) as function of time and the value of the gap:                               L          =                                    J              ⁡                              (                                  τ                  ,                                      s                    1                                                  )                                                    J              ⁡                              (                                  τ                  ,                                      s                    2                                                  )                                                    ,                            (        1        )            
where:
s1xe2x80x94is the value of the gap corresponding to the first cavity
s2xe2x80x94is the value of the gap corresponding to the second cavity.
Thus, in order to calculate the localization coefficient value, it is sufficient to use the information about the temporal behavior of the current density value as function of the gap. Further details are explained later with the reference to the figures.
Another embodiment of the method according to the invention is characterized in that a duration of the measurement voltage pulses is greater than the duration of the machining voltage pulses, the duration of the measurement voltage pulses being selected at least sufficient for a current density pulse across the machining gap to reach the global maximum. This technical measure is based on the fundamental insight that time corresponding with the global maximum of the current density pulse is a function of the absolute value of the applied voltage and the gap. For a given value of the applied voltage the time corresponding with the global maximum of the current density pulse is a direct measure of the absolute value of the gap. As will be explained in detail with reference to the figures the optimal pulse duration for the machining voltage pulses is shorter than the time corresponding to the maximum of the current density pulse. Therefore, the pulse duration for the measurement voltage pulses must be selected so that the global maximum in the current density pulse is reached. Knowing the actual gap dimensions from a previous measurement and using the information about the relation between the actual gap dimension and the corresponding optimal machining voltage pulse one can select the duration of the measurement pulse so that the resulting current density pulse across the gap reaches its global maximum. It is preferable for the polarity of the measurement voltage pulses to correspond to the polarity of the machining voltage pulses.
Another embodiment of the method according to the invention is characterized in that the value of the machining gap is corrected in case the actual value of the machining gap deviates from the predetermined value of the machining gap. Due to the fact that a predetermined value for the gap is selected to achieve a better dissolution and a surface quality of the work piece, it is preferable to correct for the deviation in the value of the gap, in case the measurements show that the actual value of the gap differs from the predetermined value. This gap correction can be performed with the electrode driving means available in the electrochemical machining device.
Another embodiment of the method according to the invention is characterized in that an interval between the machining voltage pulses is set to a value sufficient for renewal of the electrolyte in the machining gap. According to this technical measure it is assured that the operational conditions within the gap are fully recovered, it has been found that for an electrolyte comprising 5% NaCI and the operational conditions of the gap value of 30 xcexcm, applied voltage 50V and the electrolyte pressure of 300 kPa the recovery of the amplitude value of the current density is achieved after 300 xcexcs, while the recovery of the pulse form is obtained after 600 xcexcs. It has further been found that the renewal of the electrolyte is achieved after 600 xcexcs, the electrolyte flow being equal to 3 m/s.
A still further embodiment of the method according to the invention is characterized in that the value of the interval between the machining voltage pulses is derived from a system parameter including an amplitude value of the current density pulse across the machining gap and a time elapsed for the current density pulse to reach the amplitude value. It has been found that a decrease in the interval between the machining pulses leads first to a difference in the dissolution rates along the electrolyte flow followed by a creation of two separate zones. In the first zone every new machining pulse is given in a fresh electrolyte and in the second zone every new machining pulse is given into a heated electrolyte comprising a gaseous phase. This phenomenon causes pitting in the second zone, deteriorating the surface quality of the work piece. Therefore, it is concluded that in order to select the interval between the machining pulses it is more effective to use a system parameter including an amplitude value of the current density pulse across the machining gap and a time elapsed for the current density pulse to reach the amplitude value.
The invention further relates to an arrangement for electrochemically machining an electrically conductive work piece by applying electrical machining pulses between the work piece and an electrically conductive electrode while electrolyte is supplied between the work piece and the electrode.